Mechanisms of Water Oxidation Catalyzed by the cis,cis-[(bpy)₂Ru(OH₂)]₂O⁴⁺ Ion

Abstract

The cis,cis-[(bpy)(2)Ru(III)(OH(2))](2)O(4+) micro-oxo dimeric coordination complex is an efficient catalyst for water oxidation by strong oxidants that proceeds via intermediary formation of cis,cis-[(bpy)(2)Ru(V)(O)](2)O(4+) (hereafter, [5,5]). Repetitive mass spectrometric measurement of the isotopic distribution of O(2) formed in reactions catalyzed by (18)O-labeled catalyst established the existence of two reaction pathways characterized by products containing either one atom each from a ruthenyl O and solvent H(2)O or both O atoms from solvent molecules. The apparent activation parameters for micro-oxo ion-catalyzed water oxidation by Ce(4+) and for [5,5] decay were nearly identical, with DeltaH(++) = 7.6 (+/-1.2) kcal/mol, DeltaS() = -43 (+/-4) cal/deg mol (23 degrees C) and DeltaH(++) = 7.9 (+/-1.1) kcal/mol, DeltaS(++) = -44 (+/-4) cal/deg mol, respectively, in 0.5 M CF(3)SO(3)H. An apparent solvent deuterium kinetic isotope effect (KIE) of 1.7 was measured for O(2) evolution at 23 degrees C; the corresponding KIE for [5,5] decay was 1.6. The (32)O(2)/(34)O(2) isotope distribution was also insensitive to solvent deuteration. On the basis of these results and previously established chemical properties of this class of compounds, mechanisms are proposed that feature as critical reaction steps H(2)O addition to the complex to form covalent hydrates. For the first pathway, the elements of H(2)O are added as OH and H to the adjacent terminal ruthenyl O atoms, and for the second pathway, OH is added to a bipyridine ring and H is added to one of the ruthenyl O atoms.

Functional models for the oxygen-evolving complex of photosystem II

“…Since that time, 1 has become one of the most studied water oxidation catalysts [53][54][55][56][57]. Even with 25 years of study, the mechanism by which 1 oxidizes water is still under active discussion [58][59][60][61][62].…”

“…There are four main proposals, split into bimolecular and intramolecular pathways, as illustrated in Figure 6. All four mechanistic proposals are based on O-O bond formation by the bis oxo Ru (V,V) dimer that is the observed intermediate with the highest oxidation state [59].…”

“…Due to the slow exchange of ligands bound to Ru, the isotopic composition of evolved oxygen is very helpful in determining the mechanism of oxygen evolution [66]. Unfortunately, there is a conflict in the literature as to the relative isotopic composition of the evolved oxygen [59,67]. This may be a function of instrumental differences.…”

“…This may be a function of instrumental differences. The most recent measurement determined that, when 1 was enriched with 18 O at the cis-aqua positions and further oxidized in unlabeled aqueous solution, the evolved oxygen contained mostly 18 O 16 O and very little oxygen with a mass of 32 or 36 [59]. The earlier measurement found that the mixed-isotope product was less dominant [67].…”

“…In the earlier measurements where mixedlabel oxygen was not such a dominant product, all four proposed mechanisms were still in play to some extent. In addition to the four mechanisms shown in Figure 6, a new and somewhat unorthodox mechanism has recently been put forth that involves the oxidation of one of the bipyridine ligands to a diol and subsequent reduction of this diol to release oxygen and regenerate the bipyridine ligand [59]. This fifth mechanism is supported by EPR measurements that suggest that the proposed (V,V) oxidation state may in fact be a (IV,V) species with a radical on one of the bipyridine ligands [68,69] and by observations of monomeric Ru(bpy) 3 complexes trapped in zeolite cages [70].…”